목적: To investigate the neonatal outcome according to gestational age at delivery in uncomplicated twin pregnancies and to determine the optimal timing for delivery in monochorionic and dichorionic twin pregnancies. 방법: This is a retrospective cohort study of women with twin pregnancies delivered beyond 35 weeks of gestation at Samsung Medical Center from 1994 to 2013. Pregnancies complicated by one-or-more fetal death before 35 weeks` gestation, fetal chromosomal or non-chromosomal major anomalies, preeclampsia, diabetes, preterm labor, preterm rupture of membrane, abruption placenta, and placenta previa were excluded. The primary outcome was a composite morbidity defined as having one-or-more of stillbirth, admission to neonatal intensive care unit (NICU), use of ventilator, respiratory distress syndrome and neonatal mortality. The outcome was analyzed according to gestational age at delivery and chorionicity. 결과: A total of 593 twin pregnancies were included (monochorionic=141, dichorionic=452). Preterm delivery <37 weeks` gestation was performed in 41.5% (246/593), but its rate decreased over the 19 years of study period (from 63.2% in 1995-1999 to 9.8% in 2010-2013). Monochorionic twins were more commonly delivered <37 weeks` gestation (50.4% vs. 38.7%, p=0.014) compared to dichorionic twins, and they had a higher risk of NICU admission, perinatal mortality and composite morbidity. The nadir of neonatal composite morbidity in monochorionic twins was observed at 37 weeks` gestation (35.3% in 35 week, 14.0% in 36 weeks, 5.7% in 37 week, 12.5% in 38 weeks, 8.3% in ≥39 week) and the nadir of neonatal composite morbidity in dichorionic twins was observed at beyond 38 weeks` gestation (31.7% in 35 week, 7.2% in 36 weeks, 3.3% in 37 week, 1.7% in 38 weeks, 0% in ≥39 week). 결론: Our data indicates that the optimal gestational age for delivery was 37 weeks in uncomplicated monochorionic pregnancies and beyond 38 weeks in dichorionic twin pregnancies.